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Development of a piezo-optical chemical monitoring system
Sensors and Actuators B 51 (1998) 121 – 130
Development of a piezo-optical chemical monitoring system John D. Wright a,*, Florence Colin a,b, Raoul M. Sto¨ckle a,b, Paul D. Shepherd b, Telmo Labayen b, Timothy J.N. Carter b a
Centre for Materials Research, School of Physical Sciences, Uni6ersity of Kent, Canterbury, Kent CT2 7NR, UK b PiezOptic Limited, Viking House, Ellingham Way, Ashford, Kent TN23 6NF, UK Received 29 March 1998; accepted 6 May 1998
Abstract The development of an environmental and occupational chemical monitoring system based on a piezoelectric polymer film badge is described. Colour changes in reagent spots on the polymer film are measured by illumination with a flashing LED. Non-radiative decay of the excited states produced by optical absorption leads to heat and thermal expansion which stresses the polymer, generating a charge which is measured using a lock-in amplifier. The principles and the optimisation of materials and measuring system are described with reference to the development of an ammonia monitoring system. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Environment; Occupational monitoring; Sensor; Ammonia; Polyvinylidene fluoride
1. Introduction Occupational and environmental standards are generally expressed in terms of average exposures over short (e.g. 15 min) or long (e.g. 8 h) periods. Systems for obtaining data to demonstrate compliance with such standards must satisfy a set of requirements which differ substantially from those for sensors which are used for continuous monitoring. Thus, for example, a key requirement for all continuous monitoring sensors is that the response should be reversible, whereas to record a true average integrated exposure over a period of time the device should accumulate the target analyte irreversibly. The commonest way in which this has been achieved to date has been the use of charcoal absorber tubes, from which the collected material is either solvent-extracted or thermally desorbed into a GC/MS system. Although the tubes are relatively cheap, the cost of the GC/MS system is very high. An optical system using irreversible colour reactions would be preferable, but the requirement for a spectrophotometer, together with the problems of light scattering from solid samples, have hitherto made this an unattractive
* Corresponding author. Tel.: +44 1227 823519; fax: + 44 1227 827558; e-mail: [email protected]
alternative. We now report a new piezo-optical system, which measures colour in reagent spots directly by quantifying the heat generated when the spots absorb light. In this device, the reagent spot is deposited on a piezoelectric film of poled-poly-vinylidene fluoride (PVDF) [1]. Stressing such films generates an electric charge across the film, which can be measured as a voltage across the feedback resistor of an operational amplifier. In order to remove the effects of ambient noise, vibration and temperature fluctuations, two such films with opposite polarity are laminated and the outer surfaces of the composite are coated with transparent conducting indium tin oxide (ITO). Reagent spots are deposited on one side of the film and after exposure for the desired time the spots are illuminated with chopped light from an LED of appropriate colour. The resulting ac voltage is amplified using a lock-in amplifier locked to the frequency of chopping of the LED light and the signal is converted to the equivalent time-weighted exposure with reference to calibration data. The measured signal is directly proportional to the amount of absorbed light and hence to the concentration of the absorbing species. The system is thus analogous to photoacoustic spectroscopy. As with fluorescence-based systems, the signal is directly related to the target
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species concentration and is not dependent on measuring the light transmitted by the sample (which may be affected by scattering processes). Although such a device is simple in concept and the LED-based measuring system is more than two orders of magnitude cheaper than GC/MS systems, many factors must be optimised if it is to work well. These factors include choice of reagent, the matrix in which the reagent is dispersed, the spot shape and thickness and reagent concentration, the design of the badge casing to hold the coated PVDF film, means for making electrical contact to the film for measurements, choice of LED and selection of measurement conditions (e.g. chopper frequency and the need to allow for time delays between initial absorption of light and generation of the piezo-electrical signal). In this paper the optimisation of these factors is discussed, with reference to several examples and the performance of the resulting monitoring system is described.
2. Choice of reagent and matrix Colour reagents for this device must satisfy the following requirements: 1. They must be stable with respect to light, temperature, atmospheric gases, humidity and mechanical shock. 2. The colour reaction should be irreversible. 3. The colour change should be marked and either the initial or final colour should be in a spectral region giving a good match with the emission spectrum of an available LED. 4. The colour reaction should occur at ambient temperature and in a single step, not requiring sequential addition of reagents. 5. The reaction should be fast, so that transient exposures to high concentrations of analyte are faithfully recorded. 6. The reaction must be quantitative. 7. The reaction should be specific to the target analyte. 8. Effects of changes in humidity and temperature on sensitivity should be minimal. 9. The reagent should be readily available or easily synthesised. 10. The reagent must form uniform spots on ITOcoated PVDF film surfaces. 11. The reagent spot should be porous to the target analyte. 12. The reagent should either be non-toxic or be entrapped within a matrix to minimise hazards. These requirements are largely the same as those for many other optical chemical sensor reagents, so in this paper attention will be focused on those which are peculiar to this device, namely irreversible reaction and
the need to form uniform porous spots on the PVDF film. Irreversible reaction may be defined as a colour change which is not reversed during the exposure period nor in conditions of normal storage following exposure up to the time of measurement of the colour. Thus we have found that not only specific chemical reactions (such as irreversible reduction of phosphomolybdate by vinyl monomers, or diazotisation and azo coupling by nitrous acid formed from NO2) but also some processes which are in other circumstances reversible (e.g. reactions of acid–base indicators to pH changes) can be used provided the environment during sampling and storage is suitable. For example, in this paper a case history is presented of the use of a pH indicator, Rose Bengal, to monitor ammonia. Frequently reagents will not form uniform porous reactive layers on PVDF by simple evaporation of a small quantity of solution in a suitable volatile solvent. The hydrophobic nature of the PVDF/ITO surface leads to non-uniform wetting and uneven films with poor adhesion. A general solution to this problem is to entrap the reagent in a porous matrix, with the following advantages: 1. The reagent molecules are dispersed uniformly within a porous medium, providing maximum accessibility. 2. The concentration of reagent can be varied to control sensitivity and dynamic range. 3. If the pore walls are hydrophilic, the analyte reacts with reagent molecules in the presence of entrapped water, which frequently facilitates reaction. 4. The reagent and the subsequent reaction product, is physically protected. In the present work, silica sol-gel matrices have been used. Acid-catalysed hydrolysis of tetramethoxysilane in the presence of optimised amounts of water and alcohol, with the reagent dissolved in one of the components, is followed by ageing and drying [2]. The resulting product is ground into a fine powder, bound into a paste with a solution of a polymer in a volatile solvent, deposited as small spots on the PVDF film and allowed to dry.
3. Effects of spot shape and thickness and reagent concentration The signal generation process in this system involves absorption of light by the coloured reacted reagent and diffusion of the consequent heat to the surface of the PVDF film, where it causes expansion leading to the stress which generates the voltage signal. In the following discussion it will be assumed that the spot is illuminated from the side opposite to the exposed surface. As in photoacoustic spectroscopy, two key parameters are the optical absorption length (l) and the thermal diffu-
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sion length (K) [3]. Only heat generated within K of the PVDF surface can produce a signal. For large values of l (low exposures, low reagent concentrations or weakly absorbing species) some of the light will be absorbed too far from the PVDF to produce any contribution to the signal, provided the film is sufficiently thick. A similar situation would be produced if a thick film only reacted strongly in its surface layers initially. As the colour develops, or as the reacted layer thickens towards the PVDF interface, more heat will be produced within K of the surface and the observed signal will increase. Maximum sensitivity is thus expected for thin films with high concentrations of reagents, although these may have limited dynamic range. The shape of the reagent spot also affects the way in which the signal develops as exposure increases. Dome-shaped spots, with non-uniform thickness, will give rise to uneven heat transfer to the underlying surface. The centre of the spot may be of lower or higher temperature than the periphery during illumination, depending on the values of l and K in relation to the physical thickness and this uneven temperature distribution will change as exposure increases, leading to varying stresses on the PVDF and complex effects on the shape of the calibration curve. Uniform films of reagent provide theoretically simpler response behaviour, with potentially more reproducible stress patterns. All these effects are being explored, both with model systems and using real reagent spots of different geometry and reagent concentration as a function of exposure and are described elsewhere [4].
4. Design of badge casing The PVDF film with reagent spots must be supported in a housing which provides mechanical protection yet allows diffusion of the analyte to the spot and which permits electrical contact to be made to
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Fig. 1. The PiezOptic badge.
the ITO-coated surfaces for measurement while also allowing illumination of the spot. This is achieved by the use of a light hinged moulded polypropylene unit as shown in Figs. 1 and 2. The film is clamped lightly between two plastic foam pads, electrical contact being made by two conductive polymer foam pads which are penetrated by metal probes in the measuring instrument. The exposure apertures may be covered by filters to exclude potential interferents or to control sensitivity by limiting diffusion rates.
Fig. 2. Construction of the badge. (Each badge normally has five reagent spots: for clarity only one is shown.)
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Fig. 3. The badge reader.
5. Measurement of badge signal The badge reader unit (Fig. 3) consists of a set of LED sources driven at a controlled but selectable flashing rate and mark/space ratio, together with a lock-in amplifier with a variable phase lag. The phase lag results from the time between initial absorption of the light and arrival of the consequent heat at the PVDF surface and is clearly dependent on spot geometry and thickness, extent of reaction and reagent concentration. Details of this behaviour are given elsewhere [4]. Badges consisting of PVDF strips with five identical reagent spots are measured with sequential illumination of the spots, to give an exposure reading averaged over the five spots, together with data on the standard deviation. Readings are repeated several times and further averaged. Bar coding of individual badges triggers the optimised measurement conditions for the particular badge type, determined during development calibration tests and allows exposure data for specific workers to be logged. The entire measurement cycle is complete in 20 s, providing workers with instant end-of-shift feedback on their exposure in addition to logged data for the entire work force.
Scheme 1.
LEDs as shown in Fig. 4. The silver/manganese reagent involves reduction of silver ions to metallic silver with oxidation of Mn2 + to MnO2 in basic conditions. The silver produces a black colour and absorbs strongly across the visible spectrum, permitting the use of cheap high-intensity red LEDs.
6.2. Encapsulation of reagent The Rose Bengal reagent was entrapped in a porous sol-gel matrix prepared by acid-catalysed hydrolysis of tetramethoxysilane (TMOS) using 1.3 ml TMOS and 2.4 ml (water+ethanol). p-Toluenesulfonic acid was chosen as the catalyst following the work of Nakano et al. The concentration of acid catalyst affects not only the formation of the sol-gel glass but also the sensitivity of the reagent to ammonia. Increasing the concentration of acid decreases the sensitivity to ammonia since the acid must be neutralised before the indicator changes colour. Fig. 5 shows the observed responses of reagent spots prepared in identical conditions but with different amounts of acid catalyst, indicating an optimum pH of 1.57, slightly below the isoelectric point of silica sol-gels (pH 2.2). The ratio of water:ethanol (R)
6. Case history for development of an ammonia monitoring badge
6.1. Choice of reagent Rose Bengal (I) Scheme 1 [5] and silver nitrate/manganese nitrate [6] reagents were selected since they were known to satisfy most of the reagent requirements listed above. Rose Bengal is a pH indicator with a pKa of 4.5, colourless below pH 4 and red above pH 5 and thus can be used to monitor basic gases such as ammonia. The absorption spectrum of the coloured form matches well with the emission spectrum of blue –green
Fig. 4. Diffuse reflectance spectrum of the coloured form of Rose Bengal compared with the emission range of the blue – green LED.
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Fig. 5. Response of the sol-gel entrapped Rose Bengal reagent as a function of the pH of the sol-gel precursor mixture.
in the sol-gel precursor solution was also varied, showing slightly decreased ammonia sensitivity as R increased. However, the lowest practical R value was 0.25 since very low R values led to long gelation times due to the small amount of water present. Similar procedures for the silver/manganese system led to very poor responses, but preparing the sol-gel in a similar way with no entrapped reagent, followed by soaking in a solution of silver nitrate and manganese nitrate in water acidified with toluene sulfonic acid, gave material showing good response to ammonia.
6.3. Reagent concentration The concentration of Rose Bengal in the sol-gel matrix was optimised by preparing a series of composites with increasing amounts of Rose Bengal in constant mixtures of TMOS, water and ethanol. Fig. 6 shows the response after exposure to excess ammonia as a function of indicator concentration. The observed saturation of the response at high indicator concentra-
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tions is analogous to photoacoustic saturation [3], in which all the incident light is absorbed within the thermal diffusion length of the PVDF interface and converted to heat. The saturation signal magnitude is then determined by the LED output and not by the ammonia or indicator concentrations. Clearly, with lower ammonia concentrations this saturation point will not be reached and lower signals will be observed, proportional to the ammonia concentration. Higher indicator concentrations lead to a risk of saturation even in lower concentrations of ammonia and furthermore pore-blocking by the indicator itself becomes increasingly likely.
6.4. Fabrication of reagent spot The ITO coating of PVDF is damaged by acids. Hence direct deposition of the sol-gel precursor solution onto the PVDF leads to coatings which give very low responses to ammonia. If the glass-entrapped reagent composite is ground to a fine powder and bound in a porous polymer matrix (e.g. by mixing with a solution of 20 mg polyisobutylene per ml of a 1:3 hexane:toluene mixture, depositing a small volume, typically 5 ml and allowing the solvent to evaporate), reproducible thin adherent spots are obtained. Ideally such spots should be as thin as possible to minimise diffusion times for ammonia to the reagent and for heat from the reagent to the PVDF and to maximise the temperature rise and hence the observed signal for a given ammonia dose. Spin-coating the PVDF with 1 ml portions of the mixture at 5800 rpm, applying a total of ten layers, gives uniform thin coatings with rapid response and more than twice the sensitivity of the individually-deposited spots, although this deposition method is less practical for small scale use. Most subsequent tests were thus conducted using individual spots. The final optimised procedures for reagent spots for the two reagents were:
6.4.1. Rose Bengal Glass composite from 2.6 ml TMOS, 3.5 ml Rose Bengal solution (100 mg in 100 ml ethanol+ 0.2 ml 2.5 M aqueous toluenesulfonic acid), 1 ml water, 0.05 ml 2.5 M p-toluenesulfonic acid. Dry 2 days at 35°C, grind, mix 90 mg with 0.5 ml polyisobutylene solution (1 g in 10 ml hexane+ 30 ml toluene).
Fig. 6. Response of sol-gel entrapped Rose Bengal spots as a function of the indicator concentration. (Rose Bengal solution: 100 mg in 100 ml ethanol+ 0.2 ml 2.5 M p-toluenesulfonic acid. Sol-gel precursor mix: 1.7 ml TMOS, 1 ml water, 0.13 ml 2.5 M p-toluenesulfonic acid.)
6.4.2. Sil6er/manganese Glass from 5 ml TMOS, 6 ml ethanol, 3 ml water 1 ml 2.5 M p-toluenesulfonic acid, 2 days at 35°C, grind. Soak 0.5 g glass+ 0.5 ml water+ 3 ml of a solution of (3.35 g AgNO3 + 2.87 g Mn(NO3)2.7H2O in 100 ml water) for 5 days at 35°C, grind 5 min. Spots from 5 ml of solution of 100 mg soaked glass in 0.5 ml polyisobutylene solution.
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Fig. 7. Calibration curves for Rose Bengal as a function of the concentration of p-toluenesulfonic acid in the filter layer. (Circles, 0.25 M; triangles, 0.6 M; squares, 1.2 M.)
6.5. Calibration Badges prepared using Rose Bengal as described above were exposed to concentrations of 2 – 40 ppm of ammonia for 4 h periods. Saturation of the signal was observed for concentrations above 4 ppm. This is unsatisfactory since the UK occupational exposure standard is 25 ppm for 8 h or 35 ppm for 15 min. Ideally near-linear responses between 2 and 50 ppm should be available. For the silver/manganese system, the sensitivity was in this range, but a second problem emerged; the foam-plastic sponges supporting the PVDF film were found to absorb ammonia, releasing it when the supplied concentration was reduced and leading to an unacceptable drift in the reading of up to several percent per h, depending on the ammonia concentration during exposure. Both of these problems were solved by the use of glass-fibre filters (Whatman GF/G6) impregnated with 0.25 M aqueous p-toluenesulfonic acid solution to reduce the proportion of ammonia in the atmosphere which actually reached the foam pads and reagent spots. This material has an open structure, allowing diffusion of the ammonia, but also a high surface area, which allows adsorption of the acid and hence control of the proportion of ammonia allowed through. Increasing the concentration of the acid was shown to provide response to even higher ammonia concentration ranges, as shown in Fig. 7. The lower concentration of ammonia reaching the foam pads reduced the drift to insignificant levels. Fig. 8 shows a typical calibration curve for the system, obtained by exposing a series of different badges to fixed ammonia concentrations for 8 h. The scatter of points on this graph is due in part to the variations between spots in the different badges used
for each point and in part to the variation in performance of the reference badge, which at the time of these experiments had not been optimised. Nevertheless, the quality is adequate for monitoring occupational exposures. Monitoring real environments normally involves recording a time-weighted average of exposure to varying concentrations of the analyte. Tests were carried out to ensure that the badges were capable of achieving this. Two identical badges were exposed to identical total doses of ammonia, the first using a steady state concentration for the entire 8 h period while the second was exposed for a series of shorter times to a range of higher and lower concentrations. For these experiments, the responses of the badges were continuously monitored and recorded so that the build-up of colour could be seen. Fig. 9 shows a typical set of results for silver/manganese badges. Although the initial response in both cases is slow due to initial equilibration of the ammonia stream with the clean walls of the gas line and drifts are seen in periods of zero exposure due to desorption from the foam pads as discussed above, the data show that the system is able to respond rapidly to changes in dose rate and the final signals after 8 h agree to within 5%. Since this example involved concentrations from zero up to 16 times the occupational exposure standard, the test is more severe than is likely to be encountered in practice. For the Rose Bengal reagent the response to short pulses ( 1 s) of high concentrations of ammonia was shown to be faster than the responses of the silver/manganese badge in Fig. 9, stabilising within 1 min (Fig. 10) and thus ensuring adequate recording of integrated response values in variable conditions.
Fig. 8. Calibration curves for Rose Bengal using filter layer with 0.25 M p-toluenesulfonic acid, with 6000 mcd green LED (upper line) and 2000 mcd blue – green LED (lower line). Data are for ammonia in dry air at 25°C.
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Fig. 9. Comparison of the response of silver/manganese badges to steady state (A) and equivalent-TWA doses of varying concentration (B). The lower line shows the time variation of ammonia concentration used in B.
6.6. Influence of humidity Fig. 11 shows the effect of humidity on the response of the Rose Bengal system to a range of ammonia concentrations. The response decreases almost linearly as humidity increases and the effect is the same (within the error limits of measurement) independent of ammonia concentration. There are two possible explanations for this: either ammonia cannot diffuse through the filter or reagent spot so readily in humid conditions; or the ammonia reacts to form ammonium hydroxide which is strongly adsorbed on the filter layer and thus cannot reach the reagent
spot. In the test rig used, streams of ammonia in dry air and of clean humidified air are mixed in the desired proportion in a mixing chamber and passed over the test badge. To check if the reaction to form ammonium hydroxide is dominating the effect, experiments were repeated without the mixing chamber so that the ammonia had less chance of reacting with water vapour before both reached the badge. This gave higher responses. The badges therefore probably record the free ammonia concentration and the apparent humidity effect is believed to be mainly due to conversion of free ammonia to ammonium hydroxide.
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Fig. 12. Dependence of Rose Bengal response on temperature.
Fig. 10. Responses of a Rose Bengal badge to two 1-s pulses of high concentrations of ammonia.
6.7. Influence of ambient temperature Since the rates and equilibrium constants of chemical reactions, as well as the diffusion rates of analytes through membranes and the porous reagent spot matrix, are temperature dependent the response of this badge system is expected to show dependence on temperature. The accessible range of temperature is narrow, limited to temperatures above 0°C to avoid icing and below 60°C at which the poled PVDF changes its structure. Fig. 12 shows responses of Rose Bengal badges to 0, 25 and 50 ppm NH3 at 13, 25 and 58°C. As expected, at higher temperatures the response is larger and saturates at lower concentrations. Slightly smaller temperature effects were observed for the silver/ manganese badges, reflecting the fact that no filter is
needed to control the magnitude of the response of these badges so less temperature-dependent diffusion processes are involved. These problems are common to all optical chemical sensors involving chemical reactions. Temperature correction factors must therefore be applied to badges used in temperature ranges outside those for which the system has been calibrated. Incorporation of a thermally-sensitive colour spot to give a time-averaged thermal history signal would aid such correction methods.
6.8. Interferences Since both reagents depend on pH changes, interference from both acidic and basic gases might be expected. However, tests exposing partially reacted badges to 990 ppm SO2 and 490 ppm NO2 for 1 h gave no change in the reading. Exposure to 58 ppm SO2 for several h before and after an 8 h exposure to 25 ppm of
Fig. 11. Dependence of the response of a Rose Bengal badge to ammonia as a function of humidity.
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ammonia also gave no change in the normal reading for this exposure. When these experiments were repeated in 50% relative humidity, the observed decrease in response was only that seen previously in experiments with humidified clean air. This resistance to interference from acid gases is unexpected and may be due to a combination of the low pKa of Rose Bengal together with a low affinity of these acid gases at low concentrations for the already acidic spot matrix. In agreement with these arguments, interference from basic gases was observed. The magnitude of the effects of different basic gases did not correlate with their pKa values, however, as shown in Fig. 13. For both types of badge, triethylamine (pKa 10.76) and dimethylamine (pKa 10.73) gave smaller responses than ammonia. This is in contrast to the results of Nakano et al., who found larger responses for triethylamine than for ammonia and may reflect diffusion limitation for the larger amines by the sol-gel matrix used in the present work. Interference by several other basic gases was qualitatively demonstrated but not studied in detail. In most environments, ammonia is the principal alkaline atmospheric gas. In situations where other specific amines or other alkaline gases are known to dominate, badge sensitivity may be optimised by the use of diffusion filters containing different amounts of acid.
6.9. Stability Since normal atmospheres contain 2 – 30 ppb of ammonia, stored badges may deteriorate before use. Clearly, if a badge is designed to give measurable response in normal conditions over 8 h, storage in air at ambient temperature before use must be avoided. Storage in sealed polymer-coated foil bags improved shelf life by a factor of six, to over 1 month, but slow
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colouration was still observed, probably due to release of ammonia adsorbed on the foam pads in the badge into the confined space of the bag. Vacuum packing led to initial drift on opening the package as the foam pads expanded in air. Packing under carbon dioxide at room temperature led to some decrease in sensitivity over long storage times due to the acidic nature of this gas. The most effective storage method was found to be in a freezer at − 20°C, where the ambient ammonia concentration and its rate of reaction would both be minimised. Similarly, exposed badges tended to show drifts due to further reaction with atmospheric ammonia or with ammonia released from the foam pads. The latter was only a problem for the silver/manganese badges, since the acid filters used with the Rose Bengal system minimised ammonia absorption on the pads. For both systems, storage containers incorporating small amounts of solid p-toluenesulfonic acid allowed delays of several h before measurement of exposed badges, with no significant change in the reading.
6.10. Accuracy and reproducibility Reproducibility depends on the stability of the reader LEDs and amplifiers (compensated by use of a reference badge as discussed in detail elsewhere [4]) and on the reproducibility of the individual reagent spots. In this work, the reference badge used provided a standard deviation of below 2% for successive readings of the same spot. The standard deviation in readings of different badges exposed to the same concentration was up to 6.3%. Further errors arose from differences in positioning the badge in the exposure chamber (1%), errors in ammonia cylinder concentration (4%), ammonia adsorption effects on badges and test chamber walls (2%) and air flow control and measurement (1%), giving an overall figure of 9 16%. With improvements in spot deposition techniques (only manual methods were used in the present work), calibration rigs, reference badge design and measurement protocol, this overall figure could certainly be improved (e.g. each badge carries five spots and averages of repeat measurements on all five spots would clearly improve measurement statistics). The present figure already compares favourably with alternative systems [7,8].
7. Conclusions
Fig. 13. Response of Rose Bengal badges to different amines. (pKa values are: ammonia, 9.25; dimethylamine, 10.73; triethylamine, 10.76.)
The factors which determine the performance of a novel piezo-optical environmental monitoring badge system have been identified and the optimisation of the system with respect to these factors has been described using a system for monitoring ammonia as an example. This system permits monitoring of time-weighted average exposures to ammonia concentrations between 50
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ppb and 1000 ppm with an accuracy of 9 16%. Measurement of the exposed badges within 1 min with no special training is possible using a low-cost reader in which the badges are illuminated by LED sources to provide signals amplified by a lock-in amplifier. These signals are displayed as a time-weighted average exposure level and stored together with bar-coded data identifying the particular badge to provide a record of compliance with environmental or occupational exposure standards. The badges are small, light and robust and thus readily acceptable by users. Up to five different analytes can be monitored simultaneously by using an appropriate selection of reagent spots on a single badge. The system is generic and readily adaptable to new monitoring requirements. Currently systems using this principle are available for ammonia, nitrogen dioxide, glutaraldehyde, styrene, sulphur dioxide and hydrogen chloride and several others are under development.
Acknowledgements We thank the European Community for support under the ERASMUS and Leonardo da Vinci programmes and the UK Department of Trade and Industry for two SMART awards and a Teaching Company Scheme grant.
References [1] D.J. Clarke, F. Zamani-Farahani, International Patent WO 90/ 1301, November 1, 1990. [2] C.J. Brinker, G.W. Scherer, Sol-gel Science: the Physics and Chemistry of Sol-gel Processing, Academic Press, London, 1990. [3] A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy, Wiley, New York, 1980. [4] C.A. Gibson, J.D. Wright, T.J.N. Carter, Kinetic factors in the response of piezo-optical chemical monitoring devices, Sensors and Actuators 51 (1998) 238–243.
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[5] N. Nakano, Y. Kobayashi, K. Nagashima, Development of a monitoring tape for ammonia gas in air using rose Bengal, Analyst 119 (1994) 2009 – 2012. [6] K.G. Makris, U8 ber eine neue empfindliche reaktion auf ammoniak, Z. Anal. Chem. 81 (1930) 212 – 214. [7] Dra¨ger Tube Handbook, 9th ed., Dra¨gerwerk AG, Lu¨beck, Germany, 1994. [8] P.W. McConnaygaughey, E.S. McKee, I.M. Pritts, Passive colorimetric dosimeter tubes for ammonia, carbon monoxide, hydrogen sulfide, nitrogen dioxide and sulphur dioxide, Am. Ind. Hyg. Assoc. J. 46 (1985) 357 – 362.
Biographies John Wright obtained his D.Phil. at the University of Oxford in 1965 and is Reader in Materials Chemistry at the University of Kent, where he leads a research group studying materials chemistry and chemical sensors. Florence Colin obtained her Diploˆme d’Inge´nieur at the Ecole Nationale Supe´rieure de Chimie de Montpellier and her MSc at the University of Kent. She is currently a Ph.D. student and a Teaching Company Associate at PiezOptic Ltd. Raoul Sto¨ckle obtained his MSc at the University of Kent, carrying out his project work at PiezOptic Ltd. Tim Carter obtained his PhD from the University of Birmingham and was awarded the Rank Fund Prize for Opto-electronics in 1991. He is currently General Manager at PiezOptic Ltd. Paul Shepherd obtained his Ph.D. from the University of Sussex and is currently Chief Chemist at PiezOptic Ltd. Telmo Labayen obtained his MSc at the University of Kent and spent 1 year at PiezOptic Ltd. under the EC Leonardo da Vinci programme.
Development of a piezo-optical chemical monitoring system John D. Wright a,*, Florence Colin a,b, Raoul M. Sto¨ckle a,b, Paul D. Shepherd b, Telmo Labayen b, Timothy J.N. Carter b a
Centre for Materials Research, School of Physical Sciences, Uni6ersity of Kent, Canterbury, Kent CT2 7NR, UK b PiezOptic Limited, Viking House, Ellingham Way, Ashford, Kent TN23 6NF, UK Received 29 March 1998; accepted 6 May 1998
Abstract The development of an environmental and occupational chemical monitoring system based on a piezoelectric polymer film badge is described. Colour changes in reagent spots on the polymer film are measured by illumination with a flashing LED. Non-radiative decay of the excited states produced by optical absorption leads to heat and thermal expansion which stresses the polymer, generating a charge which is measured using a lock-in amplifier. The principles and the optimisation of materials and measuring system are described with reference to the development of an ammonia monitoring system. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Environment; Occupational monitoring; Sensor; Ammonia; Polyvinylidene fluoride
1. Introduction Occupational and environmental standards are generally expressed in terms of average exposures over short (e.g. 15 min) or long (e.g. 8 h) periods. Systems for obtaining data to demonstrate compliance with such standards must satisfy a set of requirements which differ substantially from those for sensors which are used for continuous monitoring. Thus, for example, a key requirement for all continuous monitoring sensors is that the response should be reversible, whereas to record a true average integrated exposure over a period of time the device should accumulate the target analyte irreversibly. The commonest way in which this has been achieved to date has been the use of charcoal absorber tubes, from which the collected material is either solvent-extracted or thermally desorbed into a GC/MS system. Although the tubes are relatively cheap, the cost of the GC/MS system is very high. An optical system using irreversible colour reactions would be preferable, but the requirement for a spectrophotometer, together with the problems of light scattering from solid samples, have hitherto made this an unattractive
* Corresponding author. Tel.: +44 1227 823519; fax: + 44 1227 827558; e-mail: [email protected]
alternative. We now report a new piezo-optical system, which measures colour in reagent spots directly by quantifying the heat generated when the spots absorb light. In this device, the reagent spot is deposited on a piezoelectric film of poled-poly-vinylidene fluoride (PVDF) [1]. Stressing such films generates an electric charge across the film, which can be measured as a voltage across the feedback resistor of an operational amplifier. In order to remove the effects of ambient noise, vibration and temperature fluctuations, two such films with opposite polarity are laminated and the outer surfaces of the composite are coated with transparent conducting indium tin oxide (ITO). Reagent spots are deposited on one side of the film and after exposure for the desired time the spots are illuminated with chopped light from an LED of appropriate colour. The resulting ac voltage is amplified using a lock-in amplifier locked to the frequency of chopping of the LED light and the signal is converted to the equivalent time-weighted exposure with reference to calibration data. The measured signal is directly proportional to the amount of absorbed light and hence to the concentration of the absorbing species. The system is thus analogous to photoacoustic spectroscopy. As with fluorescence-based systems, the signal is directly related to the target
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species concentration and is not dependent on measuring the light transmitted by the sample (which may be affected by scattering processes). Although such a device is simple in concept and the LED-based measuring system is more than two orders of magnitude cheaper than GC/MS systems, many factors must be optimised if it is to work well. These factors include choice of reagent, the matrix in which the reagent is dispersed, the spot shape and thickness and reagent concentration, the design of the badge casing to hold the coated PVDF film, means for making electrical contact to the film for measurements, choice of LED and selection of measurement conditions (e.g. chopper frequency and the need to allow for time delays between initial absorption of light and generation of the piezo-electrical signal). In this paper the optimisation of these factors is discussed, with reference to several examples and the performance of the resulting monitoring system is described.
2. Choice of reagent and matrix Colour reagents for this device must satisfy the following requirements: 1. They must be stable with respect to light, temperature, atmospheric gases, humidity and mechanical shock. 2. The colour reaction should be irreversible. 3. The colour change should be marked and either the initial or final colour should be in a spectral region giving a good match with the emission spectrum of an available LED. 4. The colour reaction should occur at ambient temperature and in a single step, not requiring sequential addition of reagents. 5. The reaction should be fast, so that transient exposures to high concentrations of analyte are faithfully recorded. 6. The reaction must be quantitative. 7. The reaction should be specific to the target analyte. 8. Effects of changes in humidity and temperature on sensitivity should be minimal. 9. The reagent should be readily available or easily synthesised. 10. The reagent must form uniform spots on ITOcoated PVDF film surfaces. 11. The reagent spot should be porous to the target analyte. 12. The reagent should either be non-toxic or be entrapped within a matrix to minimise hazards. These requirements are largely the same as those for many other optical chemical sensor reagents, so in this paper attention will be focused on those which are peculiar to this device, namely irreversible reaction and
the need to form uniform porous spots on the PVDF film. Irreversible reaction may be defined as a colour change which is not reversed during the exposure period nor in conditions of normal storage following exposure up to the time of measurement of the colour. Thus we have found that not only specific chemical reactions (such as irreversible reduction of phosphomolybdate by vinyl monomers, or diazotisation and azo coupling by nitrous acid formed from NO2) but also some processes which are in other circumstances reversible (e.g. reactions of acid–base indicators to pH changes) can be used provided the environment during sampling and storage is suitable. For example, in this paper a case history is presented of the use of a pH indicator, Rose Bengal, to monitor ammonia. Frequently reagents will not form uniform porous reactive layers on PVDF by simple evaporation of a small quantity of solution in a suitable volatile solvent. The hydrophobic nature of the PVDF/ITO surface leads to non-uniform wetting and uneven films with poor adhesion. A general solution to this problem is to entrap the reagent in a porous matrix, with the following advantages: 1. The reagent molecules are dispersed uniformly within a porous medium, providing maximum accessibility. 2. The concentration of reagent can be varied to control sensitivity and dynamic range. 3. If the pore walls are hydrophilic, the analyte reacts with reagent molecules in the presence of entrapped water, which frequently facilitates reaction. 4. The reagent and the subsequent reaction product, is physically protected. In the present work, silica sol-gel matrices have been used. Acid-catalysed hydrolysis of tetramethoxysilane in the presence of optimised amounts of water and alcohol, with the reagent dissolved in one of the components, is followed by ageing and drying [2]. The resulting product is ground into a fine powder, bound into a paste with a solution of a polymer in a volatile solvent, deposited as small spots on the PVDF film and allowed to dry.
3. Effects of spot shape and thickness and reagent concentration The signal generation process in this system involves absorption of light by the coloured reacted reagent and diffusion of the consequent heat to the surface of the PVDF film, where it causes expansion leading to the stress which generates the voltage signal. In the following discussion it will be assumed that the spot is illuminated from the side opposite to the exposed surface. As in photoacoustic spectroscopy, two key parameters are the optical absorption length (l) and the thermal diffu-
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sion length (K) [3]. Only heat generated within K of the PVDF surface can produce a signal. For large values of l (low exposures, low reagent concentrations or weakly absorbing species) some of the light will be absorbed too far from the PVDF to produce any contribution to the signal, provided the film is sufficiently thick. A similar situation would be produced if a thick film only reacted strongly in its surface layers initially. As the colour develops, or as the reacted layer thickens towards the PVDF interface, more heat will be produced within K of the surface and the observed signal will increase. Maximum sensitivity is thus expected for thin films with high concentrations of reagents, although these may have limited dynamic range. The shape of the reagent spot also affects the way in which the signal develops as exposure increases. Dome-shaped spots, with non-uniform thickness, will give rise to uneven heat transfer to the underlying surface. The centre of the spot may be of lower or higher temperature than the periphery during illumination, depending on the values of l and K in relation to the physical thickness and this uneven temperature distribution will change as exposure increases, leading to varying stresses on the PVDF and complex effects on the shape of the calibration curve. Uniform films of reagent provide theoretically simpler response behaviour, with potentially more reproducible stress patterns. All these effects are being explored, both with model systems and using real reagent spots of different geometry and reagent concentration as a function of exposure and are described elsewhere [4].
4. Design of badge casing The PVDF film with reagent spots must be supported in a housing which provides mechanical protection yet allows diffusion of the analyte to the spot and which permits electrical contact to be made to
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Fig. 1. The PiezOptic badge.
the ITO-coated surfaces for measurement while also allowing illumination of the spot. This is achieved by the use of a light hinged moulded polypropylene unit as shown in Figs. 1 and 2. The film is clamped lightly between two plastic foam pads, electrical contact being made by two conductive polymer foam pads which are penetrated by metal probes in the measuring instrument. The exposure apertures may be covered by filters to exclude potential interferents or to control sensitivity by limiting diffusion rates.
Fig. 2. Construction of the badge. (Each badge normally has five reagent spots: for clarity only one is shown.)
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Fig. 3. The badge reader.
5. Measurement of badge signal The badge reader unit (Fig. 3) consists of a set of LED sources driven at a controlled but selectable flashing rate and mark/space ratio, together with a lock-in amplifier with a variable phase lag. The phase lag results from the time between initial absorption of the light and arrival of the consequent heat at the PVDF surface and is clearly dependent on spot geometry and thickness, extent of reaction and reagent concentration. Details of this behaviour are given elsewhere [4]. Badges consisting of PVDF strips with five identical reagent spots are measured with sequential illumination of the spots, to give an exposure reading averaged over the five spots, together with data on the standard deviation. Readings are repeated several times and further averaged. Bar coding of individual badges triggers the optimised measurement conditions for the particular badge type, determined during development calibration tests and allows exposure data for specific workers to be logged. The entire measurement cycle is complete in 20 s, providing workers with instant end-of-shift feedback on their exposure in addition to logged data for the entire work force.
Scheme 1.
LEDs as shown in Fig. 4. The silver/manganese reagent involves reduction of silver ions to metallic silver with oxidation of Mn2 + to MnO2 in basic conditions. The silver produces a black colour and absorbs strongly across the visible spectrum, permitting the use of cheap high-intensity red LEDs.
6.2. Encapsulation of reagent The Rose Bengal reagent was entrapped in a porous sol-gel matrix prepared by acid-catalysed hydrolysis of tetramethoxysilane (TMOS) using 1.3 ml TMOS and 2.4 ml (water+ethanol). p-Toluenesulfonic acid was chosen as the catalyst following the work of Nakano et al. The concentration of acid catalyst affects not only the formation of the sol-gel glass but also the sensitivity of the reagent to ammonia. Increasing the concentration of acid decreases the sensitivity to ammonia since the acid must be neutralised before the indicator changes colour. Fig. 5 shows the observed responses of reagent spots prepared in identical conditions but with different amounts of acid catalyst, indicating an optimum pH of 1.57, slightly below the isoelectric point of silica sol-gels (pH 2.2). The ratio of water:ethanol (R)
6. Case history for development of an ammonia monitoring badge
6.1. Choice of reagent Rose Bengal (I) Scheme 1 [5] and silver nitrate/manganese nitrate [6] reagents were selected since they were known to satisfy most of the reagent requirements listed above. Rose Bengal is a pH indicator with a pKa of 4.5, colourless below pH 4 and red above pH 5 and thus can be used to monitor basic gases such as ammonia. The absorption spectrum of the coloured form matches well with the emission spectrum of blue –green
Fig. 4. Diffuse reflectance spectrum of the coloured form of Rose Bengal compared with the emission range of the blue – green LED.
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Fig. 5. Response of the sol-gel entrapped Rose Bengal reagent as a function of the pH of the sol-gel precursor mixture.
in the sol-gel precursor solution was also varied, showing slightly decreased ammonia sensitivity as R increased. However, the lowest practical R value was 0.25 since very low R values led to long gelation times due to the small amount of water present. Similar procedures for the silver/manganese system led to very poor responses, but preparing the sol-gel in a similar way with no entrapped reagent, followed by soaking in a solution of silver nitrate and manganese nitrate in water acidified with toluene sulfonic acid, gave material showing good response to ammonia.
6.3. Reagent concentration The concentration of Rose Bengal in the sol-gel matrix was optimised by preparing a series of composites with increasing amounts of Rose Bengal in constant mixtures of TMOS, water and ethanol. Fig. 6 shows the response after exposure to excess ammonia as a function of indicator concentration. The observed saturation of the response at high indicator concentra-
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tions is analogous to photoacoustic saturation [3], in which all the incident light is absorbed within the thermal diffusion length of the PVDF interface and converted to heat. The saturation signal magnitude is then determined by the LED output and not by the ammonia or indicator concentrations. Clearly, with lower ammonia concentrations this saturation point will not be reached and lower signals will be observed, proportional to the ammonia concentration. Higher indicator concentrations lead to a risk of saturation even in lower concentrations of ammonia and furthermore pore-blocking by the indicator itself becomes increasingly likely.
6.4. Fabrication of reagent spot The ITO coating of PVDF is damaged by acids. Hence direct deposition of the sol-gel precursor solution onto the PVDF leads to coatings which give very low responses to ammonia. If the glass-entrapped reagent composite is ground to a fine powder and bound in a porous polymer matrix (e.g. by mixing with a solution of 20 mg polyisobutylene per ml of a 1:3 hexane:toluene mixture, depositing a small volume, typically 5 ml and allowing the solvent to evaporate), reproducible thin adherent spots are obtained. Ideally such spots should be as thin as possible to minimise diffusion times for ammonia to the reagent and for heat from the reagent to the PVDF and to maximise the temperature rise and hence the observed signal for a given ammonia dose. Spin-coating the PVDF with 1 ml portions of the mixture at 5800 rpm, applying a total of ten layers, gives uniform thin coatings with rapid response and more than twice the sensitivity of the individually-deposited spots, although this deposition method is less practical for small scale use. Most subsequent tests were thus conducted using individual spots. The final optimised procedures for reagent spots for the two reagents were:
6.4.1. Rose Bengal Glass composite from 2.6 ml TMOS, 3.5 ml Rose Bengal solution (100 mg in 100 ml ethanol+ 0.2 ml 2.5 M aqueous toluenesulfonic acid), 1 ml water, 0.05 ml 2.5 M p-toluenesulfonic acid. Dry 2 days at 35°C, grind, mix 90 mg with 0.5 ml polyisobutylene solution (1 g in 10 ml hexane+ 30 ml toluene).
Fig. 6. Response of sol-gel entrapped Rose Bengal spots as a function of the indicator concentration. (Rose Bengal solution: 100 mg in 100 ml ethanol+ 0.2 ml 2.5 M p-toluenesulfonic acid. Sol-gel precursor mix: 1.7 ml TMOS, 1 ml water, 0.13 ml 2.5 M p-toluenesulfonic acid.)
6.4.2. Sil6er/manganese Glass from 5 ml TMOS, 6 ml ethanol, 3 ml water 1 ml 2.5 M p-toluenesulfonic acid, 2 days at 35°C, grind. Soak 0.5 g glass+ 0.5 ml water+ 3 ml of a solution of (3.35 g AgNO3 + 2.87 g Mn(NO3)2.7H2O in 100 ml water) for 5 days at 35°C, grind 5 min. Spots from 5 ml of solution of 100 mg soaked glass in 0.5 ml polyisobutylene solution.
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Fig. 7. Calibration curves for Rose Bengal as a function of the concentration of p-toluenesulfonic acid in the filter layer. (Circles, 0.25 M; triangles, 0.6 M; squares, 1.2 M.)
6.5. Calibration Badges prepared using Rose Bengal as described above were exposed to concentrations of 2 – 40 ppm of ammonia for 4 h periods. Saturation of the signal was observed for concentrations above 4 ppm. This is unsatisfactory since the UK occupational exposure standard is 25 ppm for 8 h or 35 ppm for 15 min. Ideally near-linear responses between 2 and 50 ppm should be available. For the silver/manganese system, the sensitivity was in this range, but a second problem emerged; the foam-plastic sponges supporting the PVDF film were found to absorb ammonia, releasing it when the supplied concentration was reduced and leading to an unacceptable drift in the reading of up to several percent per h, depending on the ammonia concentration during exposure. Both of these problems were solved by the use of glass-fibre filters (Whatman GF/G6) impregnated with 0.25 M aqueous p-toluenesulfonic acid solution to reduce the proportion of ammonia in the atmosphere which actually reached the foam pads and reagent spots. This material has an open structure, allowing diffusion of the ammonia, but also a high surface area, which allows adsorption of the acid and hence control of the proportion of ammonia allowed through. Increasing the concentration of the acid was shown to provide response to even higher ammonia concentration ranges, as shown in Fig. 7. The lower concentration of ammonia reaching the foam pads reduced the drift to insignificant levels. Fig. 8 shows a typical calibration curve for the system, obtained by exposing a series of different badges to fixed ammonia concentrations for 8 h. The scatter of points on this graph is due in part to the variations between spots in the different badges used
for each point and in part to the variation in performance of the reference badge, which at the time of these experiments had not been optimised. Nevertheless, the quality is adequate for monitoring occupational exposures. Monitoring real environments normally involves recording a time-weighted average of exposure to varying concentrations of the analyte. Tests were carried out to ensure that the badges were capable of achieving this. Two identical badges were exposed to identical total doses of ammonia, the first using a steady state concentration for the entire 8 h period while the second was exposed for a series of shorter times to a range of higher and lower concentrations. For these experiments, the responses of the badges were continuously monitored and recorded so that the build-up of colour could be seen. Fig. 9 shows a typical set of results for silver/manganese badges. Although the initial response in both cases is slow due to initial equilibration of the ammonia stream with the clean walls of the gas line and drifts are seen in periods of zero exposure due to desorption from the foam pads as discussed above, the data show that the system is able to respond rapidly to changes in dose rate and the final signals after 8 h agree to within 5%. Since this example involved concentrations from zero up to 16 times the occupational exposure standard, the test is more severe than is likely to be encountered in practice. For the Rose Bengal reagent the response to short pulses ( 1 s) of high concentrations of ammonia was shown to be faster than the responses of the silver/manganese badge in Fig. 9, stabilising within 1 min (Fig. 10) and thus ensuring adequate recording of integrated response values in variable conditions.
Fig. 8. Calibration curves for Rose Bengal using filter layer with 0.25 M p-toluenesulfonic acid, with 6000 mcd green LED (upper line) and 2000 mcd blue – green LED (lower line). Data are for ammonia in dry air at 25°C.
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Fig. 9. Comparison of the response of silver/manganese badges to steady state (A) and equivalent-TWA doses of varying concentration (B). The lower line shows the time variation of ammonia concentration used in B.
6.6. Influence of humidity Fig. 11 shows the effect of humidity on the response of the Rose Bengal system to a range of ammonia concentrations. The response decreases almost linearly as humidity increases and the effect is the same (within the error limits of measurement) independent of ammonia concentration. There are two possible explanations for this: either ammonia cannot diffuse through the filter or reagent spot so readily in humid conditions; or the ammonia reacts to form ammonium hydroxide which is strongly adsorbed on the filter layer and thus cannot reach the reagent
spot. In the test rig used, streams of ammonia in dry air and of clean humidified air are mixed in the desired proportion in a mixing chamber and passed over the test badge. To check if the reaction to form ammonium hydroxide is dominating the effect, experiments were repeated without the mixing chamber so that the ammonia had less chance of reacting with water vapour before both reached the badge. This gave higher responses. The badges therefore probably record the free ammonia concentration and the apparent humidity effect is believed to be mainly due to conversion of free ammonia to ammonium hydroxide.
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Fig. 12. Dependence of Rose Bengal response on temperature.
Fig. 10. Responses of a Rose Bengal badge to two 1-s pulses of high concentrations of ammonia.
6.7. Influence of ambient temperature Since the rates and equilibrium constants of chemical reactions, as well as the diffusion rates of analytes through membranes and the porous reagent spot matrix, are temperature dependent the response of this badge system is expected to show dependence on temperature. The accessible range of temperature is narrow, limited to temperatures above 0°C to avoid icing and below 60°C at which the poled PVDF changes its structure. Fig. 12 shows responses of Rose Bengal badges to 0, 25 and 50 ppm NH3 at 13, 25 and 58°C. As expected, at higher temperatures the response is larger and saturates at lower concentrations. Slightly smaller temperature effects were observed for the silver/ manganese badges, reflecting the fact that no filter is
needed to control the magnitude of the response of these badges so less temperature-dependent diffusion processes are involved. These problems are common to all optical chemical sensors involving chemical reactions. Temperature correction factors must therefore be applied to badges used in temperature ranges outside those for which the system has been calibrated. Incorporation of a thermally-sensitive colour spot to give a time-averaged thermal history signal would aid such correction methods.
6.8. Interferences Since both reagents depend on pH changes, interference from both acidic and basic gases might be expected. However, tests exposing partially reacted badges to 990 ppm SO2 and 490 ppm NO2 for 1 h gave no change in the reading. Exposure to 58 ppm SO2 for several h before and after an 8 h exposure to 25 ppm of
Fig. 11. Dependence of the response of a Rose Bengal badge to ammonia as a function of humidity.
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ammonia also gave no change in the normal reading for this exposure. When these experiments were repeated in 50% relative humidity, the observed decrease in response was only that seen previously in experiments with humidified clean air. This resistance to interference from acid gases is unexpected and may be due to a combination of the low pKa of Rose Bengal together with a low affinity of these acid gases at low concentrations for the already acidic spot matrix. In agreement with these arguments, interference from basic gases was observed. The magnitude of the effects of different basic gases did not correlate with their pKa values, however, as shown in Fig. 13. For both types of badge, triethylamine (pKa 10.76) and dimethylamine (pKa 10.73) gave smaller responses than ammonia. This is in contrast to the results of Nakano et al., who found larger responses for triethylamine than for ammonia and may reflect diffusion limitation for the larger amines by the sol-gel matrix used in the present work. Interference by several other basic gases was qualitatively demonstrated but not studied in detail. In most environments, ammonia is the principal alkaline atmospheric gas. In situations where other specific amines or other alkaline gases are known to dominate, badge sensitivity may be optimised by the use of diffusion filters containing different amounts of acid.
6.9. Stability Since normal atmospheres contain 2 – 30 ppb of ammonia, stored badges may deteriorate before use. Clearly, if a badge is designed to give measurable response in normal conditions over 8 h, storage in air at ambient temperature before use must be avoided. Storage in sealed polymer-coated foil bags improved shelf life by a factor of six, to over 1 month, but slow
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colouration was still observed, probably due to release of ammonia adsorbed on the foam pads in the badge into the confined space of the bag. Vacuum packing led to initial drift on opening the package as the foam pads expanded in air. Packing under carbon dioxide at room temperature led to some decrease in sensitivity over long storage times due to the acidic nature of this gas. The most effective storage method was found to be in a freezer at − 20°C, where the ambient ammonia concentration and its rate of reaction would both be minimised. Similarly, exposed badges tended to show drifts due to further reaction with atmospheric ammonia or with ammonia released from the foam pads. The latter was only a problem for the silver/manganese badges, since the acid filters used with the Rose Bengal system minimised ammonia absorption on the pads. For both systems, storage containers incorporating small amounts of solid p-toluenesulfonic acid allowed delays of several h before measurement of exposed badges, with no significant change in the reading.
6.10. Accuracy and reproducibility Reproducibility depends on the stability of the reader LEDs and amplifiers (compensated by use of a reference badge as discussed in detail elsewhere [4]) and on the reproducibility of the individual reagent spots. In this work, the reference badge used provided a standard deviation of below 2% for successive readings of the same spot. The standard deviation in readings of different badges exposed to the same concentration was up to 6.3%. Further errors arose from differences in positioning the badge in the exposure chamber (1%), errors in ammonia cylinder concentration (4%), ammonia adsorption effects on badges and test chamber walls (2%) and air flow control and measurement (1%), giving an overall figure of 9 16%. With improvements in spot deposition techniques (only manual methods were used in the present work), calibration rigs, reference badge design and measurement protocol, this overall figure could certainly be improved (e.g. each badge carries five spots and averages of repeat measurements on all five spots would clearly improve measurement statistics). The present figure already compares favourably with alternative systems [7,8].
7. Conclusions
Fig. 13. Response of Rose Bengal badges to different amines. (pKa values are: ammonia, 9.25; dimethylamine, 10.73; triethylamine, 10.76.)
The factors which determine the performance of a novel piezo-optical environmental monitoring badge system have been identified and the optimisation of the system with respect to these factors has been described using a system for monitoring ammonia as an example. This system permits monitoring of time-weighted average exposures to ammonia concentrations between 50
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ppb and 1000 ppm with an accuracy of 9 16%. Measurement of the exposed badges within 1 min with no special training is possible using a low-cost reader in which the badges are illuminated by LED sources to provide signals amplified by a lock-in amplifier. These signals are displayed as a time-weighted average exposure level and stored together with bar-coded data identifying the particular badge to provide a record of compliance with environmental or occupational exposure standards. The badges are small, light and robust and thus readily acceptable by users. Up to five different analytes can be monitored simultaneously by using an appropriate selection of reagent spots on a single badge. The system is generic and readily adaptable to new monitoring requirements. Currently systems using this principle are available for ammonia, nitrogen dioxide, glutaraldehyde, styrene, sulphur dioxide and hydrogen chloride and several others are under development.
Acknowledgements We thank the European Community for support under the ERASMUS and Leonardo da Vinci programmes and the UK Department of Trade and Industry for two SMART awards and a Teaching Company Scheme grant.
References [1] D.J. Clarke, F. Zamani-Farahani, International Patent WO 90/ 1301, November 1, 1990. [2] C.J. Brinker, G.W. Scherer, Sol-gel Science: the Physics and Chemistry of Sol-gel Processing, Academic Press, London, 1990. [3] A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy, Wiley, New York, 1980. [4] C.A. Gibson, J.D. Wright, T.J.N. Carter, Kinetic factors in the response of piezo-optical chemical monitoring devices, Sensors and Actuators 51 (1998) 238–243.
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[5] N. Nakano, Y. Kobayashi, K. Nagashima, Development of a monitoring tape for ammonia gas in air using rose Bengal, Analyst 119 (1994) 2009 – 2012. [6] K.G. Makris, U8 ber eine neue empfindliche reaktion auf ammoniak, Z. Anal. Chem. 81 (1930) 212 – 214. [7] Dra¨ger Tube Handbook, 9th ed., Dra¨gerwerk AG, Lu¨beck, Germany, 1994. [8] P.W. McConnaygaughey, E.S. McKee, I.M. Pritts, Passive colorimetric dosimeter tubes for ammonia, carbon monoxide, hydrogen sulfide, nitrogen dioxide and sulphur dioxide, Am. Ind. Hyg. Assoc. J. 46 (1985) 357 – 362.
Biographies John Wright obtained his D.Phil. at the University of Oxford in 1965 and is Reader in Materials Chemistry at the University of Kent, where he leads a research group studying materials chemistry and chemical sensors. Florence Colin obtained her Diploˆme d’Inge´nieur at the Ecole Nationale Supe´rieure de Chimie de Montpellier and her MSc at the University of Kent. She is currently a Ph.D. student and a Teaching Company Associate at PiezOptic Ltd. Raoul Sto¨ckle obtained his MSc at the University of Kent, carrying out his project work at PiezOptic Ltd. Tim Carter obtained his PhD from the University of Birmingham and was awarded the Rank Fund Prize for Opto-electronics in 1991. He is currently General Manager at PiezOptic Ltd. Paul Shepherd obtained his Ph.D. from the University of Sussex and is currently Chief Chemist at PiezOptic Ltd. Telmo Labayen obtained his MSc at the University of Kent and spent 1 year at PiezOptic Ltd. under the EC Leonardo da Vinci programme.